This invention relates to a vertical apparatus for heat treating a semiconductor substrate.
A heat-treating apparatus for a semiconductor substrate applied until now involves horizontal ard vertical types. The vertical heat-treating apparatus is classified into two types:
the type wherein an opening for enabling a wafer-carrying boat to be taken into and out of the reactor is formed in the upper portion of the heat-treating apparatus; and
the type wherein said opening is perforated in the lower portion of said apparatus.
In the vertical heat-treating apparatus whose upper portion is perforated by an opening, an air curtain for releasing a large volume of gas is provided near the opening. When the wafer-carrying boat is taken into and out of the reactor, the curtain is intended to suppress the backward flow of the outside air into the reactor at the opening. However, even the air curtain provided near the opening fails to completely stop the backward flow of the outside air. Further, the application of a large volume of gas is accompanied with many drawbacks, for instances, the introduction of large volumes of highly heated gas into the cooling room. The vertical heat-treating apparatus whose lower portion is perforated by an opening has the merit that since the outside air having a lower temperature than the gas held in the reactor has a greater density, the backward flow of the outside air into the reactor can be prevented by the influx of a small volume of gas into the reactor. With this type of a semiconductor substrate-heating apparatus, the boat is surrounded by a cylinder to shut off the outside air, or in some cases, the reaction gas is let to run down the reactor. At any rate, gas has to flow through the reactor in a larger amount than prescribed. With the vertical type heating apparatus, however, it is practical and sufficient to let gas flow in a far smaller volume than in the horizontal type apparatus. Now let it be assumed that the opening has a diameter of 300 mm. Then it is necessary to let gas run at the rate of 20 l/min. Both the heat-treating apparatus whose upper portion is perforated by an opening and the type whose lower portion is provided with an opening are characterized in that the region uniformly heated to an elevated temperature is spaced apart from the opening at a short distance. When the gas flow rate is prominently increased, the temperature near the opening rises. Therefore, care should be taken about the quality and heat resistance of the walls of the heat treating apparatus of a semiconductor substrate. Discharge of large volumes of highly heated gas into a clean room raises various problems including heat load. Consequently there is a demand to minimize the gas flow rate and prevent the back flow of open air.
FIGS. 1A and 1B are sectional views of a vertical semiconductor substrate heat treating apparatus whose lower portion is perforated by an opening. Reactor 1 comprises an opening. The top wall of reactor 1 is provided with reaction gas inlet 3. Gas outlet 4 is provided at the lower part of the side wall of reactor 1. Aperture 2 of reactor 1 is fitted with vertically movable cap 5. Aperture 2 is opened or closed according to the movement of cap 5. Boat rest 6 is set on cap 5. Positioned on boat rest 6 is boat 7 designed to horizontally hold a plurality of wafers (not shown). Boat rest 6 is intended to hold boat 7 effectively in a uniformly heated region a to shut off heat released from reactor 1, thereby suppressing temperature rise in cap 5. Cylindrical jacket 8 surrounding boat 7 is mounted on boat rest 6. Gas outlet 9 is provided in the lower portion of one side wall of jacket 8 to prevent reaction gas from being retained in jacket 8.
A vertical semiconductor substrate heat treating apparatus constructed as described above is operated in the following manner. Reaction gas enters the reactor at inlet 3. The reactor gas taken into reactor 1 flows around boat 7 and then through outlet 9 of jacket 8 and further through outlet 4 of reactor 1 and finally into the open air (FIG. 1A).
After heat treatment, cap 5 is let to fall (FIG. 1B). At this time aperture 2 of reactor 1 is exposed to outside air. Boat 7 is shut off from the outside air by jacket 8. If an interstice between reactor 1 and jacket 8 is extremely narrow and has an extremely small sectional area, reaction gas is also released through outlet 9 of jacket 8. Therefore, the outside air is prevented from entering jacket 8. Practically, however, it is necessary to allow a certain interstice between reactor 1 and jacket 8 in order to facilitate the vertical movement of jacket 8. Therefore, reaction gas is fully released through an interstice between reactor 1 and jacket 8 and outlet 4 of reactor 1. As a result, it sometime happens that reaction gas finals to fully flow to outlet 9, thus giving rise to the backward flow of outside air, and consequently sometimes deteriorating the property of a finished semiconductor element, particularly the voltage property of a flat band.